The mere prospect of a Donald Trump presidency is already hurting America's global image, according to a high-level group of diplomats and elected officials across Europe and the U.S..

"A madman," "clown," and "freak politics," is how opinion-leaders in POLITICO's fourth Transatlantic Caucus summed up the real estate tycoon and his controversy-filled campaign for the Republican nomination.

The view was overwhelming: 83 percent of Caucus participants said Trump’s candidacy damages America’s standing in the world.

"Donald Trump's rhetoric and simple-minded approach is dangerous to American standing in the world," said one Caucus member. "He offers nothing good for the U.S. or the world at large."

POLITICO polled 76 policy insiders on both sides of the Atlantic who spoke candidly on a range of issues on condition they be quoted anonymously. They included NATO Deputy Secretary-General Alexander Vershbow, six countries’ ambassadors to the EU, former World Bank President Paul Wolfowitz, President of the Eurogroup Working Group Thomas Wieser, and Republican U.S. Congressman Reid Ribble. A full list of participants is here.

Concerns about Trump mirrored the group's anxieties about the divisive rhetoric from European politicians. The Caucus rated "the rise of populist parties and threats to democracy" as the Continent's third greatest threat after the migration crisis and Islamist terrorism — forces that have thrown into doubt the visa-free Schengen agreement, a cornerstone of the European Union.

In the U.S., Trump has called for a ban on all Muslims coming to the country, while accusing Mexico of sending rapists across the border.

One Caucus member said Trump's candidacy offered only "moronic solutions to complex problems."

A CNN/ORC poll Wednesday showed Trump running well ahead of the rest of the Republican field, with 39 percent support. However he is trailing Senator Ted Cruz in the crucial state of Iowa, where the first contest of the primary campaign will be held February 1. On Monday, a Reuters/Ipsos poll found Trump would lose to Democrat Hillary Clinton if the presidential election were held now.

"The pre-primary period says very little about the real options and plans for the presidential candidates," one Caucus member argued. "All it shows at the moment is how divided the Republican base is."

Even if Trump loses in his presidential bid, participants worried about the larger implications of his campaign and the effect if might have over the long term on the American political landscape.

"It is not his candidacy, but the support he receives," a Caucus member said, "like some extreme politicians in EU member states, including France."

Following the November 13 attacks in Paris, National Front leader Marine Le Pen called for a halt to the reception and distribution of migrants in France. She has sought to distance herself from Trump, however, arguing his proposed ban on Muslims is too extreme.

Neither she nor any member of her party prevailed in French regional elections this month, but a strong showing in the first round of voting signaled she could still feature in the run-off to become French president in 2017.

One Caucus member argued the main problem is the EU's German-led open-door policy towards asylum-seekers.

"That is unsustainable," the respondent said. "Either the EU changes this idealistic, well-meaning, but unworkable granting of universal rights, or EU states will simply abrogate Schengen."

A madman. Just one of the comments Donald Trump got in our poll of top policy makers (Getty)

Trump has made immigration one of his key campaign issues, and has not been shy about criticizing Europe's approach to the issue, which he claims has fed a terrorism threat. He said recently that parts of Paris and London are so radicalized police refuse to go there.

He called German Chancellor Angela Merkel's decision to welcome Syrian refugees "insane," while embracing Vladimir Putin whom he called "a strong leader."

The Russia President returned the compliment, calling Trump “a very bright and talented man.”

British Prime Minister David Cameron labeled the proposed ban on Muslims as "stupid and wrong," while J.K. Rowling said Trump was worse than Voldemort -- the villain of her Harry Potter series.

A petition to ban him from entering the U.K. has attracted more than half a million signatures.